I am trying to remember a classic rock song. Somewhere in the middle or towards the end, all the music stops, and there is this cool spacy synthy effect: from very high notes, a spiral down into very low notes, almost percussiony.

I'm glad I could help. I used to have the Edgar Winter LP "They Only Come Out at Night", which includes both "Frankenstein" and "Free Ride". Won it in a contest in high school in the 1970's. The cover photograph always freaked me out, but I did enjoy the music!